As it is known, many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages formed from a continuous tube of packaging material made from a longitudinally sealed web.
The packaging material has a multilayer structure comprising a strong, stiff base layer, which may comprise fibrous material, such as paper, or material such as mineral-filled polypropylene. The base layer is covered on both sides with layers of heat-seal plastic material, such as polyethylene films; in the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of oxygen-barrier material, such as an aluminium foil or an ethyl vinyl alcohol (EVOH) foil, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material defining the inner face of the package eventually contacting the food product.
As it is known, packages of this sort are produced on fully automatic packaging machines, on which the web of packaging material is unwound off a reel and fed through a sterilizing unit, in which it is typically sterilized by immersion in a bath of liquid sterilizing agent, such as a concentrated hydrogen peroxide and water solution.
More specifically, the sterilizing unit comprises a bath filled, in use, with the sterilizing agent, into which the web is fed continuously. The bath conveniently comprises two parallel vertical branches connected at the bottom to define a U-shaped path long enough to allow sufficient time to treat the packaging material. For effective, relatively fast treatment, thus enabling a reduction in the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature, e.g. of around 70° C.-80° C.
The sterilizing unit also defines an aseptic environment connected to the outlet of the bath, and in which the web of packaging material is dried and subsequently folded and sealed longitudinally to form a vertical tube, which is then filled continuously with the food product for packaging.
More specifically, in the aseptic environment, the web is treated to eliminate any residual sterilizing agent, the amount of which permitted in the packaged food product is governed by strict regulations (the maximum amount permitted being in the region of a fraction of one part per million).
The above treatment normally comprises a preliminary operation, whereby the drops on the packaging material are removed mechanically, and air drying.
Preliminary removal of the drops may be performed, for example, by means of a pair of squeeze rollers conveniently located close to the inlet of the aseptic environment; the packaging material is fed between the rollers and comes out still covered with a film of sterilizing agent, but with no macroscopic drops.
Drying may be performed using air knives directed onto the opposite faces of the web of packaging material, supplied with sterile air, and for evaporating any leftover traces of sterilizing agent.
Before leaving the aseptic environment, the web is folded into a cylinder and sealed longitudinally to form a continuous vertical tube in known manner. The tube of packaging material, in effect, forms an extension of the aseptic environment, and is filled continuously with the pourable food product, and then fed to a (transverse) form-and-seal unit for forming the individual packages, and in which the tube is gripped and sealed between pairs of jaws to form pillow packs.
The pillow packs are separated by cutting the sealed portions between the packs, and are then fed to a final folding station where they are folded mechanically into the finished form.
In some known solutions, the packages coming out of the form-and-seal unit are already provided with reclosable opening devices. In these cases, the opening devices are pre-applied, e.g. injection molded directly, to the web of packaging material before the latter is supplied to the sterilizing unit.
Packaging machines of the type described above are used widely and satisfactorily in a wide range of food industries to produce sealed packages from a web of packaging material. Performance of the sterilizing units of such machines, in particular, ensures ample compliance with regulations governing sterility of the packages and the amount of residual sterilizing agent in the finished packages.
Within the industry, however, a demand for further improvements is felt, particularly in view of the continual increase in the output rate of the packaging machines and of the impact that such increase may have on the solutions based on the application of the opening devices to the packaging material before the latter is fed to the sterilizing unit.
As a matter of fact, continually increasing the output rate obviously reduces the time available to remove all the residual sterilizing agent from each portion of the web of packaging material travelling through the aseptic environment, and especially on the pre-applied opening devices, which, having a more complex geometry than the web, tend to form some sorts of traps for the residual sterilizing agent.